The role of encapsulation and host age in the clearance of Haemophilus influenzae bacteremia.

Nonimmune rats given intravenous inoculations of 10(8) encapsulated Haemophilus influenzae type b cleared the bacteria at an exponential rate for 10 min; thereafter, the bacteremia plateaued at approximately 10(6) organisms/ml of blood. With 10(8) unencapsulated organisms, the initial clearance rate was significantly faster (P less than 0.001) and was complete by 30 min. The rate of clearance of a mutant strain of H. influenzae type b containing 0.1% as much capsular polysaccharide as the wild type was significantly faster (P less than 0.01), and H. influenzae type b from which a portion of the capsule had been removed physically had an intermediate clearance pattern. The addition of 1-1,000 mug of capsular polysaccharide to an inoculum of 10(8) organisms did not alter the clearance of the capsular polysaccharide-deficient mutant. The quantity of bacteria cleared during the 30 min after inoculation increased with the age of the animal. The initial bacterial clearance rate, corrected for animal and organ weight, also increased with age. These data are consistent with the proposal that physically integrated capsular polymer increases the virulence of H. influenzae type b by rendering it resistant to clearance from the bloodstream and that there may be an age-related increase in phagocytic activity that is reflected in increased clearance.